In rock drilling, drilling equipment is occasionally caught in the drill hole. If the drilling equipment cannot be pulled out of the drilled hole, the shank and some drill rods need to be left in the drill hole. The drill hole cannot be used after this, and a new hole needs to be drilled beside it. Naturally, such situations are to be avoided, since the loss of drilling equipment and the drilling of a new hole cause considerable extra costs. A feed apparatus of the rock drill is usually used to pull out drilling equipment stuck in a drill hole while at the same time having the percussion device hit the drilling equipment. The problem is, however, that when the drilling equipment is pulled backward, the shank moves away from the impact point and the percussion device cannot produce hard enough impacts to free the stuck drilling equipment from the hole. Solutions have been developed for the above-mentioned problem, in which the shank is pulled to the impact point during the freeing. This is typically arranged by forming a pulling piston to the shank or around it, the pulling piston being arranged through a pressure medium to pull the shank toward the percussion piston in relation to the front end of the rock drill, i.e. toward the designed impact point. Such solutions are disclosed for instance in U.S. Pat. Nos. 4,109,734, 4,718,500, and 5,002,136.
Further, WO publication 98/42481 discloses a solution, in which cylinder spaces parallel to the shank are formed around the shank, each having a pulling piston arranged to it. A pulling sleeve is arranged between the confronting faces of the pulling pistons and the shank to transmit a pulling force from the pistons to the shank.
A problem with known lifting piston constructions is that impacts of the percussion piston also hit the pulling element during normal rock drilling, because at least in upward drilling, the pulling element can due to gravity move against the shank. In present solutions, the operating life of the pulling element is short due to high impact stress. Further, if the pulling element supports the shank during drilling, the impacts of the percussion piston cause a pulling force in the drilling equipment at least when the drilling equipment is not sufficiently supported against rock. As generally known, a pulling force causes the threads between drilling components to open and wears thread joints.